Re: [PATCH v9 4/8] KVM: Use gfn instead of hva for mmu_notifier_retry

Chao Peng
2022-11-08T07:21:10Z

On Fri, Nov 04, 2022 at 10:29:48PM +0000, Sean Christopherson wrote:
> On Fri, Nov 04, 2022, Chao Peng wrote:
> > On Thu, Oct 27, 2022 at 11:29:14AM +0100, Fuad Tabba wrote:
> > > Hi,
> > >
> > > On Tue, Oct 25, 2022 at 4:19 PM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > > >
> > > > Currently in mmu_notifier validate path, hva range is recorded and then
> > > > checked against in the mmu_notifier_retry_hva() of the page fault path.
> > > > However, for the to be introduced private memory, a page fault may not
> > > > have a hva associated, checking gfn(gpa) makes more sense.
> > > >
> > > > For existing non private memory case, gfn is expected to continue to
> > > > work. The only downside is when aliasing multiple gfns to a single hva,
> > > > the current algorithm of checking multiple ranges could result in a much
> > > > larger range being rejected. Such aliasing should be uncommon, so the
> > > > impact is expected small.
> > > >
> > > > It also fixes a bug in kvm_zap_gfn_range() which has already been using
> > >
> > > nit: Now it's kvm_unmap_gfn_range().
> >
> > Forgot to mention: the bug is still with kvm_zap_gfn_range(). It calls
> > kvm_mmu_invalidate_begin/end with a gfn range but before this series
> > kvm_mmu_invalidate_begin/end actually accept a hva range. Note it's
> > unrelated to whether we use kvm_zap_gfn_range() or kvm_unmap_gfn_range()
> > in the following patch (patch 05).
>
> Grr, in the future, if you find an existing bug, please send a patch. At the
> very least, report the bug.

Agreed, this can be sent out separately from this series.

> The APICv case that this was added for could very
> well be broken because of this, and the resulting failures would be an absolute
> nightmare to debug.

Given the apicv_inhibit should be rare, the change looks good to me.
Just to be clear, your will send out this fix, right?

Chao

>
> Compile tested only...
>
> --
> From: Sean Christopherson <seanjc@google.com>
> Date: Fri, 4 Nov 2022 22:20:33 +0000
> Subject: [PATCH] KVM: x86/mmu: Block all page faults during
> kvm_zap_gfn_range()
>
> When zapping a GFN range, pass 0 => ALL_ONES for the to-be-invalidated
> range to effectively block all page faults while the zap is in-progress.
> The invalidation helpers take a host virtual address, whereas zapping a
> GFN obviously provides a guest physical address and with the wrong unit
> of measurement (frame vs. byte).
>
> Alternatively, KVM could walk all memslots to get the associated HVAs,
> but thanks to SMM, that would require multiple lookups. And practically
> speaking, kvm_zap_gfn_range() usage is quite rare and not a hot path,
> e.g. MTRR and CR0.CD are almost guaranteed to be done only on vCPU0
> during boot, and APICv inhibits are similarly infrequent operations.
>
> Fixes: edb298c663fc ("KVM: x86/mmu: bump mmu notifier count in kvm_zap_gfn_range")
> Cc: stable@vger.kernel.org
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f81539061d6..1ccb769f62af 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6056,7 +6056,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
>
> write_lock(&kvm->mmu_lock);
>
> - kvm_mmu_invalidate_begin(kvm, gfn_start, gfn_end);
> + kvm_mmu_invalidate_begin(kvm, 0, -1ul);
>
> flush = kvm_rmap_zap_gfn_range(kvm, gfn_start, gfn_end);
>
> @@ -6070,7 +6070,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> gfn_end - gfn_start);
>
> - kvm_mmu_invalidate_end(kvm, gfn_start, gfn_end);
> + kvm_mmu_invalidate_end(kvm, 0, -1ul);
>
> write_unlock(&kvm->mmu_lock);
> }
>
> base-commit: c12879206e47730ff5ab255bbf625b28ade4028f
> --